Global plan Reinforcement learning I: –prediction –classical conditioning –dopamine Reinforcement learning II: –dynamic programming; action selection –Pavlovian misbehaviour –vigor Chapter 9 of Theoretical Neuroscience (thanks to Yael Niv)

2 Conditioning Ethology –optimality –appropriateness Psychology –classical/operant conditioning Computation –dynamic progr. –Kalman filtering Algorithm –TD/delta rules –simple weights Neurobiology neuromodulators; amygdala; OFC nucleus accumbens; dorsal striatum prediction: of important events control: in the light of those predictions

= Conditioned Stimulus = Unconditioned Stimulus = Unconditioned Response (reflex); Conditioned Response (reflex) Animals learn predictions Ivan Pavlov

Animals learn predictions Ivan Pavlov very general across species, stimuli, behaviors

But do they really? temporal contiguity is not enough - need contingency 1. Rescorla’s control P(food | light) > P(food | no light)

But do they really? contingency is not enough either… need surprise 2. Kamin’s blocking

But do they really? seems like stimuli compete for learning 3. Reynold’s overshadowing

Theories of prediction learning: Goals Explain how the CS acquires “value” When (under what conditions) does this happen? Basic phenomena: gradual learning and extinction curves More elaborate behavioral phenomena (Neural data) P.S. Why are we looking at old-fashioned Pavlovian conditioning?  it is the perfect uncontaminated test case for examining prediction learning on its own

error-driven learning: change in value is proportional to the difference between actual and predicted outcome Assumptions: 1.learning is driven by error (formalizes notion of surprise) 2.summations of predictors is linear A simple model - but very powerful! –explains: gradual acquisition & extinction, blocking, overshadowing, conditioned inhibition, and more.. –predicted overexpectation note: US as “special stimulus” Rescorla & Wagner (1972)

how does this explain acquisition and extinction? what would V look like with 50% reinforcement? eg –what would V be on average after learning? –what would the error term be on average after learning? Rescorla-Wagner learning

how is the prediction on trial (t) influenced by rewards at times (t-1), (t-2), …? Rescorla-Wagner learning recent rewards weigh more heavily why is this sensible? learning rate = forgetting rate! the R-W rule estimates expected reward using a weighted average of past rewards

Summary so far Predictions are useful for behavior Animals (and people) learn predictions (Pavlovian conditioning = prediction learning) Prediction learning can be explained by an error-correcting learning rule (Rescorla-Wagner): predictions are learned from experiencing the world and comparing predictions to reality Marr:

But: second order conditioning animals learn that a predictor of a predictor is also a predictor of reward!  not interested solely in predicting immediate reward animals learn that a predictor of a predictor is also a predictor of reward!  not interested solely in predicting immediate reward phase 1: phase 2: test: ? what do you think will happen? what would Rescorla-Wagner learning predict here?

lets start over: this time from the top Marr’s 3 levels: The problem: optimal prediction of future reward what’s the obvious prediction error? what’s the obvious problem with this? want to predict expected sum of future reward in a trial/episode (N.B. here t indexes time within a trial)

lets start over: this time from the top Marr’s 3 levels: The problem: optimal prediction of future reward want to predict expected sum of future reward in a trial/episode Bellman eqn for policy evaluation

lets start over: this time from the top Marr’s 3 levels: The problem: optimal prediction of future reward The algorithm: temporal difference learning temporal difference prediction error  t compare to:

17 prediction error no predictionprediction, rewardprediction, no reward TD error VtVt R RL

Summary so far Temporal difference learning versus Rescorla-Wagner derived from first principles about the future explains everything that R-W does, and more (eg. 2 nd order conditioning) a generalization of R-W to real time

Back to Marr’s 3 levels The problem: optimal prediction of future reward The algorithm: temporal difference learning Neural implementation: does the brain use TD learning?

Dopamine Dorsal Striatum (Caudate,Putamen) VentralTegmental Area SubstantiaNigra Amygdala NucleusAccumbens (Ventral Striatum) Prefrontal Cortex Dorsal Striatum (Caudate,Putamen) VentralTegmental Area SubstantiaNigra Amygdala NucleusAccumbens (Ventral Striatum) Prefrontal Cortex Dorsal Striatum (Caudate,Putamen) VentralTegmental Area SubstantiaNigra Amygdala NucleusAccumbens (Ventral Striatum) Prefrontal Cortex Dorsal Striatum (Caudate,Putamen) VentralTegmental Area SubstantiaNigra Amygdala NucleusAccumbens (Ventral Striatum) Prefrontal Cortex Parkinson’s Disease  Motor control + initiation? Intracranial self-stimulation; Drug addiction; Natural rewards  Reward pathway?  Learning? Also involved in: Working memory Novel situations ADHD Schizophrenia …

Role of dopamine: Many hypotheses Anhedonia hypothesis Prediction error (learning, action selection) Salience/attention Incentive salience Uncertainty Cost/benefit computation Energizing/motivating behavior

22 dopamine and prediction error no predictionprediction, rewardprediction, no reward TD error VtVt R RL

prediction error hypothesis of dopamine Tobler et al, 2005 Fiorillo et al, 2003 The idea: Dopamine encodes a reward prediction error

prediction error hypothesis of dopamine model prediction error measured firing rate Bayer & Glimcher (2005) at end of trial:  t = r t - V t (just like R-W)

what drives the dips? why an effect of reward at all? –Pavlovian influence Matsumoto & Hikosaka (2007)

what drives the dips? rHab -> rSTN RMTg (predicted R/S) Matsumoto & Hikosaka (2007) Jhou et al, 2009 first other

Risk Experiment < 1 sec 0.5 sec You won 40 cents 5 sec ISI 19 subjects (dropped 3 non learners, N=16) 3T scanner, TR=2sec, interleaved 234 trials: 130 choice, 104 single stimulus randomly ordered and counterbalanced 2-5sec ITI 5 stimuli: 40¢ 20¢ 0 / 40¢ 0¢ 5 stimuli: 40¢ 20¢ 0 / 40¢ 0¢

Neural results: Prediction Errors what would a prediction error look like (in BOLD)?

Neural results: Prediction errors in NAC unbiased anatomical ROI in nucleus accumbens (marked per subject*) * thanks to Laura deSouza raw BOLD (avg over all subjects) can actually decide between different neuroeconomic models of risk

30 generalization

31 generalization

other paradigms inhibitory conditioning transreinforcer blocking motivational sensitivities backwards blocking –Kalman filtering downwards unblocking primacy as well as recency (highlighting) –assumed density filtering

Summary of this part: prediction and RL Prediction is important for action selection The problem: prediction of future reward The algorithm: temporal difference learning Neural implementation: dopamine dependent learning in BG  A precise computational model of learning allows one to look in the brain for “hidden variables” postulated by the model  Precise (normative!) theory for generation of dopamine firing patterns  Explains anticipatory dopaminergic responding, second order conditioning  Compelling account for the role of dopamine in classical conditioning: prediction error acts as signal driving learning in prediction areas

Striatum and learned values Striatal neurons show ramping activity that precedes a reward (and changes with learning!) (Schultz) startfood (Daw)

Phasic dopamine also responds to… Novel stimuli Especially salient (attention grabbing) stimuli Aversive stimuli (??) Reinforcers and appetitive stimuli induce approach behavior and learning, but also have attention functions (elicit orienting response) and disrupt ongoing behaviour. → Perhaps DA reports salience of stimuli (to attract attention; switching) and not a prediction error? (Horvitz, Redgrave)